The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Wellbore isolation during stimulation (for example by fracturing, acidizing, and acid fracturing) is performed by a variety of methods within the oilfield industry. One of the traditional approaches involves the use of ball sealers, which are meant to seal the perforations in the casing and prevent fluid in the wellbore from flowing through the perforations into the formation.
Ball sealers are typically spheres designed to seal perforations that are capable of accepting fluid, and thus divert reservoir treatments to other portions of a target zone. Ball sealers are slightly larger than the perforations and are incorporated in the treatment fluid and pumped with it. They are carried to the perforations by the fluid flow, seat in the holes, and are held there by differential pressure. The effectiveness of this type of mechanical diversion requires keeping the balls in place and completely blocking the perforations, and depends on factors such as the differential pressure across the perforation, the geometry of the perforation, and physical characteristics of the ball sealers.
If the ball sealers have reached the entrance hole of the perforation tunnels and a pressure up event does not occur, this may indicate that there is significant flow past the ball sealers and into the formation. In such cases, it is important that pumping is stopped to ensure any proppant fracturing fluid or other treatment fluid is not washed away. Conversely, a premature pressure event may be realized if the proppant of the fracturing fluid screens out. In such cases, the ball sealers and proppant slurry may lie across a subsequent zone to be stimulated. This can result in an immediate ball out (balls landing), premature screenout (proppant slurry) or stuck perforation guns in the subsequent zone to be stimulated.
The accurate displacement of ball sealers is especially important when conducting “just in time perforating” (JITP) operations. In such operations, multiple zones or intervals are sequentially perforated, with fracturing fluids being introduced into the formation to stimulate the perforated zone while the perforating gun remains in the well. Ball sealers are introduced into the wellbore to seal the perforations so that a subsequent zone may be treated. When a pressure event is observed as a result of the ball sealers sealing perforations in a previously fractured zone, the next zone to be treated is perforated. The JITP operation allows multiple zones or intervals to be quickly and efficiently treated in a single, continuous pumping operation. Inaccurate displacement of the ball sealers impedes the operation and prevents subsequent zones from being treated until the ball sealers are accurately placed.
Ball sealer displacement is conventionally measured through the displaced volume of fluid that is introduced into the wellbore, as measured at the surface. This can lead to severe inaccuracies in displacement of the ball sealers. This can be due to a variety of different factors. These may include inaccuracies in the exact volume of fluid used in the treatment, inaccuracies in the exact volume of fluid between the first and last ball sealers being pumped, inaccuracies in the standard pump volume and inaccuracies due to ball sealer movement within the displacement fluid while traveling downhole.
Because of the inaccuracies in displacement of ball sealers using conventional methods improvements are needed.